1. Field of the Invention
The present invention relates to the field of projection displays and more specifically to the automated measurement of such displays.
2. Description of the Related Art
The convergence and focus of projection displays having more than one spatial light modulator (SLM) are typically determined subjectively by an operator. As a result, repeatability and tight tolerances in converging and focusing many projectors are difficult to accomplish. The results often depend on the skill and motivation of the person making the adjustments.
FIG. 1 illustrates the convergence issue in a three-micromirror projection display (used for example only). The three micromirrors, each dedicated to one of the three primary colors of light (red, green, and blue), respectively are embedded within the optical system of the projector. The images from these three micromirrors are combined by means of combining prisms and as a result, require mechanical alignment so that corresponding pixels from each array lay exactly on top of each other. FIG. 1a shows the same pixel from each of the red 1, green 2, and blue 3 micromirrors. In this out-of-convergence example, where the green 2 pixel is the reference, the red 1 and blue 3 pixels are shifted relative to the reference green 2 pixel as shown in Table 1 below.
TABLE 1XyR−0.4−0.2G 0  0 B+0.2+0.2It is clear from the figure that this system needs to be converged, at least in the area of the observed pixel. This is best illustrated by the picture of FIG. 1b, which shows the unaligned red 1 and blue 3 pixels relative to the reference green 2 pixel. (Note: these show up as fuzzy edges in this B/W illustration, but as mis-aligned color pixels in a color photo). In the actual color picture, the non-convergence is best observed along the edges of the pixel where a blue leading edge is seen at the top and right edge of the pixel and a red trailing edge is seen at the bottom and left edge of the pixel. Typically, an operator would adjust the x and y locations of the red 1 and blue 3 micromirrors until the three images align with on another and the system is converged, resulting in a white image.
Focus is another parameter where the adjustment by an operator is often made subjectively. This parameter is more complicated to properly adjust, with many variables involved. For example, brightness can affect the focus significantly. In a projection system, focus is usually accomplished by means of the projection lens, which can be either a zoom or fixed focal length lens. FIG. 2 illustrates a row and column of pixels from a three-micromirror projection system, which is clearly out-of-focus. Typically, the projector's operator will adjust the projection lens to provide the best focus, according to his desires.
FIG. 3 shows a row and column of pixels that have been both converged and focused manually by an operator. This shows the image properly converged, with the red, green, and blue pixels being properly aligned so as to appear as one pixel, white in color, and with sharp edges around both the pixel and around the hole in the center of the pixel. This hole in the center of the pixel is where the support post for the micromirror attaches to the mirror.
What is needed is an objective method for convergence and focus criteria along with a measuring tool for implementing the method. This method needs to reflect the human element since the human eye is the final arbitrator in a display application. The invention disclosed herein addresses this need by means of both a method and a tool.